https://chromium.googlesource.com/chromium/src/+log/52.0.2723.0..52.0.2729.0?pretty=fuller&n=10000

This is overlay fullscreen video I think. Which means it's WontFix unless we want janky rotations.

Ah, yeah, you're probably right. Do you know whether WMPA shows posters in fullscreen? If not, then I'd agree that it's a wontfix for now.

When I run a test with the flags "--disable-unified-media-pipeline" and "--force-fieldtrials=UnifiedMediaPipelineTrial/Disabled", (which I assume is WMPA), fullscreening the media element results in the poster image disappearing: https://av-analysis.corp.google.com/#/get-testpass-details/2/Video%20Stack/130806

Just to make sure this is clear: this is a regression. So we're trading this ability to display poster images for the ability to reduce fullscreen jankiness. Seems like a reasonable exchange, but I just wanted to be clear.

watk - does adding a check for shouldDisplayVideo() in LayoutVideo::additionalCompositingReasons fix the issue?

not sure that it doesn't have unintended side-effects, though, like "turning off the SV optimization".  the SV was being created properly, but i didn't get a chance to dig deeper to see if it was using the correct drawing path.

Good call. Making the condition this works:
    if (shouldDisplayVideo() && element->isFullscreen() && element->usesOverlayFullscreenVideo())

WMPA has seemingly had this bug for no reason. We might have to turn off OverlayFullscreenVideo because of a different but I'm looking at, so we might not need to make this change just yet.